An aqueous emulsion of polytetrafluoroethylene (hereinafter referred to as “PTFE”) is obtainable by emulsion-polymerizing tetrafluoroethylene (hereinafter referred to as “TFE”). Such an aqueous emulsion of PTFE (hereinafter referred to as a “PTFE aqueous emulsion”) is subjected to stabilization treatment by adding e.g. a nonionic surfactant, as the case requires, and concentrated to obtain a PTFE aqueous dispersion. And, after adding various compounding ingredients, such a dispersion is used for application to various coatings or application for impregnation. Further, a fine powder of PTFE (hereinafter referred to as a “PTFE fine powder”) is obtainable by exerting a shearing force to a PTFE aqueous emulsion to coagulate PTFE microparticles, followed by drying. The PTFE fine powder is molded by a method such as paste extrusion molding and then used for various applications.
In the emulsion polymerization of TFE, a fluorinated emulsifier is usually employed. As such a fluorinated emulsifier, ammonium perfluorooctanoate (structural formula: CF3(CF2)6COONH4, hereinafter referred to as “APFO”) having 8 carbon atoms wherein the main chain is composed solely of carbon atoms, is commonly used. However, APFO is not present in the natural world and is a hardly decomposable substance, and further, it has been pointed out that its biological accumulation property is high, and it has been proposed to suppress its discharge from the environmental aspect.
Whereas, many techniques have been proposed to use a fluorinated carboxylic acid containing etheric oxygen atoms in its molecule, or its salt, as a fluorinated emulsifier having a low biological accumulation property (e.g. Patent Documents 1 to 3).
Patent Document 1 discloses a fluorinated emulsifier of the formula CF3—(OCF2)m—O—CF2—X (wherein X is a carboxylic acid or its salt, and m is from 1 to 6) and/or CF3—O—(CF2)3—(OCF(CF3)—CF2)z—O-L-Y (wherein Y is a carboxylic acid or its salt, z is from 0 to 3, and L is —CF(CF3)—, —CF2— or —CF2CF2—). Further, in Example 1 thereof, CF3—O—C3F6—OCF(CF3)COONH4 is disclosed, and relatively high molecular weight PTFE having a standard specific gravity (hereinafter referred to as “SSG”) of 2.154 is produced. However, in a Comparative Example using APFO, SSG is 2.148, whereby it is suggested that with the above fluorinated emulsifier, the molecular weight can hardly be increased. Further, the solid content concentration is less than 10%, whereby the ability as a surfactant is presumed to be low as compared with APFO.
Patent Document 2 discloses a process for producing PTFE by using at least 800 g/mol of a dispersant wherein a fluoropolyether acid or its salt, and a fluorinated emulsifier having a main chain length of at most 6 atoms are used in combination. In Examples 5 to 9 therein, a fluoropolyether having a molecular weight of about 2,100 and C3F7—O—CF(CF3)—COONH4 are used in combination. Although a few percent of PTFE is coagulated and deposited during the polymerization, it is possible to obtain a PTFE aqueous emulsion having a solid content concentration of from 28.0 to 29.9%, and a sufficient stability of the PTFE aqueous emulsion can be secured. However, SSG of PTFE is 2.177, whereby it is presumed difficult to increase the molecular weight of PTFE. Further, 20% of the dispersant used, is a fluoropolyether having a molecular weight of about 2,100. Such a fluoropolyether has a molecular weight higher than APFO, whereby there is a concern about its accumulation in vivo or its discharge in vitro as well as its residual amount in a PTFE product or its influence to a molded product.
Patent Document 3 discloses a method for emulsion-polymerizing TFE by means of such a fluorinated emulsifier that among compounds represented by Rf1—O—(Rf2O)n—Rf3—COOM (wherein Rf1 is a C1-3 fluoroalkyl group, each of Rf2 and Rf3 which may be the same or different, is a C1-3 fluoroalkylene group, M is anyone of H, K, Na or NH4, and n is an integer of 0 or more), at least two compounds different in the value of n are present so that the average value is from 2 to 4. If the fluoroalkyl group in the fluorinated emulsifier becomes a long chain, the residual amount in PTFE increases, although the surface activating performance may be improved, and if it becomes a short chain, the surface activating performance tends to be inadequate, although the residual amount in PTFE may be small. It is disclosed that by combining fluorinated emulsifiers having long and short chains, the fluorinated emulsifiers can easily be removed from PTFE, and further, the surface activating performance can sufficiently be obtainable. In Examples, the solid content concentration is 33.0%, and a sufficient stability of a PTFE aqueous emulsion is secured. On the other hand, SSG is 2.189, whereby it is presumed difficult to attain a high molecular weight of PTFE.
The residual amount in PTFE is influenced by the chain length of the long chain component and the mixing ratio to the short chain component. In Examples in Patent Document 3, CF3O(CF2O)4CF2COONH4 contained in an amount of 10% has a length of the main chain excluding the carboxy group being 11 atoms as the sum of carbon and oxygen atoms, and thus, is likely to have a high residual nature as compared with APFO having a length of the main chain being 7 atoms.
Thus, with the fluorinated emulsifier presumed to have a lower residual nature in PTFE than APFO and low biological accumulation, the surface activating performance is low, the stability of the PTFE aqueous emulsion is inadequate, and various problems may occur, and it is difficult to obtain PTFE having a high molecular weight. By using a high molecular weight fluoropolyether or a fluorinated emulsifier having a longer chain in combination, the surface activating performance will be improved, but there is a concern about the problem of the residual nature in PTFE or the influence to a molded product.
Patent Document 4 discloses that by copolymerizing a (perfluoroalkyl)ethylene to TFE at the initial stage of emulsion polymerization of TFE so that the content of polymerized units derived from the (perfluoroalkyl)ethylene will be from 0.005 to 0.05 mol %, it is possible to obtain a PTFE fine powder excellent in extrusion processability, whereby uniform stretch processing is possible, and a high strength porous material can be obtained. It is disclosed that as a fluorinated emulsifier, a C7-9 ammonium perfluoroalkane carboxylate is preferably employed, but in each of Examples, APFO is used. Thus, since APFO is used as a fluorinated emulsifier, there has been a problem that the environmental load is large. Further, Patent Document 4 discloses nothing about the heat resistance which is an important element in the molding of a stretched porous material of PTFE. Further, there is no disclosure relating to the stability of the PTFE aqueous emulsion. Therefore, it has been not known whether or not it is possible to obtain a PTFE aqueous emulsion whereby the stability of the PTFE aqueous emulsion is sufficiently high to such an extent not to impair the processability or moldability, and it is possible to prepare a molded product excellent in heat resistance, in a case where a fluorinated emulsifier having a lower surface activating performance than APFO is employed.